Animation smoothness and correctness when exporting
I resolved the first issue by deleting all of my auto-generated keyframes as they'd recorded positions and angles for all of my model. I then re-created four keyframes manually (right-click the relevant property, in this case the W of the rotation, and select 'Make single keyframe'). Selecting them all in a graph editor window then allowed me to:

a) Make a cycle (Shift-e)
b) Apply linear interpolation (T)

This produced a smooth rotation in Blender, although something strange happened to make the direction the opposite of what I'd configured, so I reversed the slope in the graph editor.

Exporting the result still produced only linear interpolation between the four keyframes in SceneKit, so I used the graph editor again to make an actual keyframe at every interpolated point (a total of 40). I'm sure this isn't the ideal solution, but it does work.